The automated deposition of coating materials, such as adhesives, onto the surfaces of workpieces is commonly performed through the use of program controlled devices such as robot mounted discharge guns. The devices which support the guns are programmed to move the guns in such a way in relation to the workpiece surface so as to scribe a predetermined deposition pattern onto a series of articles or workpieces. In such devices, a robot control program establishes the tool speed while a gun nozzle control attempts to control a volume discharge. rate which will maintain the bead width of the coating material at a constant or other predetermined thickness as the tool speed in relation to the surface of the workpiece varies. The tool speed is a term defined in the art of robotics as the linear or scaler speed at which the point of application of coating material to the workpiece moves across the workpiece surface. The tool speed is most often established by controlling the motion of the discharge gun through the articulation of a robot arm on which the gun is mounted. The volume rate of the discharge of the coating material is established by the pressure of the fluid in the gun and nozzle opening of the discharge gun.
The tool speed may in some applications be controlled for the most part to remain uniform, but even in such applications, the tool speed will nonetheless necessarily change at certain portions of the coating path, particularly where changes in the direction of the tool occur. When the tool speed changes, unless a compensating variation is made in the volume discharge rate of the coating material from the nozzle, the linear deposition of material will be non-uniform. A uniform deposition, or bead size, is usually an important objective to be achieved in the coating process. This is particularly true in the case of adhesives deposited on a substrate or workpiece.
For example, in the application of elastic seals to door panels, such as the doors of refrigerators and automobiles, a closed irregular path is scribed by the nozzle of an adhesive discharge gun along the surface of a door panel as it is moved past the robot on an assembly line. The robot scribes the loop at a speed which changes at corners and along curved portions of the path. To maintain a constant bead size, which is a goal in the adhesive application process, the aperture of the gun nozzle is often programmed to vary at least in inverse proportion to the tool speed. Due to the nature of the materials of which the adhesive are made, however, such control of the bead size has been less than satisfactory for maintaining a constant bead size in that the flow of the material has tended to be, under such circumstances, non-linear.
Due to the nature of their applications, coating materials are often developed with special flow properties. In the case of adhesives, for example, it is desirable that the material, once applied to the workpiece surface, remain in place and not flow under the influence of gravity or otherwise until the part to be glued is in place. Such an adhesive material must, however, be flowable from the nozzle and onto the workpiece surface so that it can be rapidly and accurately applied during the adhesive application process. As a result, these fluids have an initial resistance to flow at low pressures, but as they begin to flow under the influence of the pressure imposed on the material at the gun that resistance declines and they thereupon flow more freely.
The characteristic of a fluid which is non-flowable when deposited on a workpiece but which becomes increasingly more flowable the more rapidly it is ejected through a nozzle is known as shear thinning, the thinning effect caused by the internal shearing stress produced in the fluid passing through the nozzle. Materials possessing this shear-thinning property, unfortunately, do not thin only as they begin to flow, but do so in differing degrees depending on the rate of at which they are discharged through a nozzle. Even more unfortunately, many factors such as nozzle configuration, material composition, temperature and other factors, not all of which are controllable or predictable, influence the degree to which shear-thinning will occur. Furthermore, it is not only these specially developed adhesives, but many other dispensed fluid materials which possess this property to some degree. In addition, due to this cause and for other reasons, many fluids are not ideal, possessing non-linear, or non-Newtonian flow characteristics which may cause their resistance to flow to either increase or decrease as the forces which move them change.
Heretofore, there has not been an effective or adequate apparatus or method to compensate for the shear thinning effects and other non-linear flow characteristics of materials during a controlled deposition process. Accordingly, there exists a need to provide a solution to the problem of the non-linear relationship between flowability of dispensed materials and volume flow rates and other fluid dispensing parameters.